Cable Broadband: How It Works

By  Malcolm Taylor

Cable networks were originally established as unidirectional networks to deliver television and radio stations into the customers' home. Cable provided a high quality alternative to the aerial radio and television broadcasting that was often subject to interference. The old cable networks were fully coaxial cable based.

In mainland Europe, the earliest deployments started in the 1930s. Until the 1990s, there were thousands of small networks all over Europe but most of these are now consolidated into larger cable operators.

In the UK, cable networks started to emerge from the mid 1980s, following a policy decision by the government to liberalise the telecommunications market and create ‘infrastructure competition’ to BT, which was subsequently privatised. Although some UK cable networks were established during the latter half of the 1980s, it wasn’t until the early 1990s that cable network build really accelerated and in a 6-7 year period, over 50% of UK households were passed by new cable networks.

In mainland Europe, cable operators needed to upgrade their networks from unidirectional to two-way capability and invested extensively in fibre. In UK, because of the later start, extensive fibre was deployed from the outset.

As a result, most current cable networks contain significant levels of optical fibre, often to less than 100 metres from the customers' premises.  The final connection into the customers’ premises is coaxial cable. Consequently, the name 'hybrid fibre-coax' (HFC) network is used to describe the majority of modern cable networks. Based on this network structure, in addition to the traditional broadcast services, cable operators can now offer broadband Internet services in excess of 100Mbps.

How it works

The cable network comprises a number of elements – the headend, the fibre and coaxial cabling to the customers’ premises and the individual customer’s terminal equipment.

The headend is where the broadcast content is received, either from a satellite or a local TV antenna or sometimes via a direct fibre link from a studio.  The headend processes and assembles the content for onward delivery to the customer. It also connects with other  network and service providers.

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In a modern HFC cable plant, fibre optic cables carry the content (radio frequency signals) as light (optical signals) from the headend to optical nodes in the various neighbourhoods served by the cable network. The node converts the optical signals back to RF signals and the local part of the cable network distributes the RF signals to the customer, over the coaxial cable. Typically, local nodes serve between 500 to 1000 customers’ premises.

In addition, the HFC architecture enables the delivery of signals that originate in customers’ premises back to the headend. This two-way capability supports the provision of interactive audio, video and data services.

The local coaxial (or drop) cable is connected to consumer electronics equipment, often referred to as CPE (customer premises equipment), inside the home. This equipment (such as television sets, set-top boxes, cable modems and personal video recorders) processes the cable signals and enables subscribers to view, record, and interact with those services.

Most cable operators provide set-top boxes and cable modems (that connect to the HFC plant to provide always-on, high-speed access to the Internet) as part of a subscription package.

In addition, telephone services are offered on cable networks using a “telephony over IP” protocol  (which is based on EuroPacketCable 1.0/1.5 standards). Now, many cable modems incorporate the telephony function and are increasingly wireless routers.

How much capacity?

Typically, HFC cable networks carry multiple television channels, radio and telephone services, video on demand (VOD) and broadband Internet services using the range of UHF spectrum. In most cases, this extends to 862MHz but cable networks will operate up to 1GHz as more capacity is required to meet growing demands from customers for bandwidth hungry services.

In terms of broadcast services, multiple channels are available, and these are comparable to those delivered by direct to home (DTH) satellite.

Broadband Internet services are provided using a technology known as EuroDOCSIS, the latest version of which (EuroDOCSIS 3.0) allows data speeds of 160 Mbps downstream and 120 Mbps upstream, which is, at least four times faster than the previous EuroDOCSIS version. These speeds are achieved by the ‘bundling’ or combining of a number of channels.

As EuroDOCSIS 3.0 has no limit in how many channels it can bundle, the speed for data communications via cable will progressively increase to multiples of 160Mbps and 120Mbps.

EuroDOCSIS 3.0 also accommodates the increased demand for IP addresses by integrating the new Internet Protocol version 6 (IPv6). The demand for more IP addresses is generated by an array of new Internet enabled devices (laptops, PVRs, mobile phones, etc.).

Strengths and weaknesses

The key strength of a modern HFC cable network, when compared to other mainstream broadband technologies, is the extensive use of fibre optic cabling, deep into the local community, which allows the provision of significantly greater numbers of broadcast services as well as very high speed broadband, based on the latest generation of EuroDOCSIS technology referred to above.

The combination of fibre to local nodes and coaxial cable drops to the customers’ premises, as opposed to the twisted pair cables in older incumbent telephone networks, means that significantly more bandwidth is available. At present, the coaxial drop is adequate but the current HFC structure also provides a very good base for cable operators to extend fibre into the home to further increase capacity.

Another potential benefit of cable networks is that, with local headends, more localised broadcast and other services can be provided.

Next-generation broadband

Any network, large or small, particularly those serving local communities, has to provide a range of services that customers seek – particularly in a competitive broadcast and broadband market. To make a wide range of services available to customers, all networks have to interconnect with other networks and with content and service providers. In this respect, new networks need to deploy technology that will support the transfer of content and services across network interfaces and also look to find areas of mutual interest with other network operators.

As far as the UK is concerned, the existing mainstream cable network operator, Virgin Media, reaches between 50–55% of households, compared to BT’s universal coverage.  New community projects can offer the opportunity for cable to extend its reach, whilst customers within the community can benefit from the service range that cable technology offers.